Recently, the research and development of a so-called high density optical disk in which, by using a blue laser light of wavelength of about 400 nm, the recording density of the optical information recording medium (optical disk) is increased, and the memory capacity is enlarged, are advanced. As a standard of the high density optical disk, for example, a standard in which an image side numerical aperture (NA) of an objective lens is about 0.85, the protective-substrate thickness is about 0.1 mm, or a standard in which NA and the protective substrate thickness are controlled to about 0.65 and about 0.6 mm, which are about same degree as a conventional DVD (Digital Versatile Disk), is well-known. In the following description, a high density optical disk in which NA is about 0.85, a protective substrate thickness is about 0.1 mm, is expressed as “BD (Blu-ray Disc), and a high density optical disk in which NA is about 0.65, a protective substrate thickness is about 0.6 mm, is expressed as “AOD (Advanced Optical Disc)”. AOD is sometimes called as “HD-DVD (High Density-DVD)”.
Then, various technologies relating to an optical pick-up apparatus having a compatibility of such a high density optical disk with a conventionally widely used optical disk such as DVD or CD (Compact Disk), are proposed.
Hereupon, the wavelengths λ1/λ2/λ3 of the light fluxes used for AOD/DVD/CD are respectively about 400 nm/about 650 nm/about 780 nm, and the protective substrate thickness t1/t2/t3 are respectively about 0.1 mm/about 0.6 mm/about 1.2 mm.
For attaining the compatibility between such a plurality of kinds of optical disks, it is necessary that the aberration generated due to the difference of wavelengths or protective substrate thickness is corrected while the light amounts of the light flux used for each of optical disks are assured, and the technology to provide the diffractive structure on the optical surface of the optical element constituting the optical pick-up apparatus is disclosed. For example, the invention disclosed in Japanese Un-examined Patent Application Publication (hereinafter, referred to as JP-A) No. 8-249707, is an optical pick-up apparatus for DVD/CD compatibility, and when a hologram collimator is arranged just before the objective lens, the reproduction of DVD is conducted by using the first-order diffraction light, and the reproduction of CD is conducted by using the non-diffraction light.
Further, to intend the size reduction of the optical pick-up apparatus, there is a case where a light source unit in which 2 light sources for emitting the light fluxes of different wavelengths are packaged into one package, is used, and in this case, for the purpose that the incident angles on the objective lens are made different for 2 kinds of light fluxes, the technology in which a coupling lens or a collimator is arranged, is widely known. For example, the invention disclosed in JP-A No. 2001-236680, is the optical pick-up apparatus for DVD/CD compatibility using the light source unit, and in which, by providing the diffractive structure on the coupling lens, the chromatic aberration of the coupling lens itself generated due to the wavelength difference between 2 kinds of light fluxes is corrected.
However, in the invention disclosed in the above patent document JP-A No. 8-249707, because the reproduction of CD is conducted by using the non-diffraction light, it does not has a sufficient correction function as compared to a case where the diffraction light is used.
On the one hand, to attain the compatibility by the high density optical disk/DVD/CD, it is necessary that, not only the correction of the aberration generated due to the difference of the wavelength or the protective substrate thickness as described above, but also the chromatic aberration generated due to the wavelength variation of the light flux used for the high density optical disk is corrected, it is difficult that aberration is sufficiently corrected by the technology disclosed in the above patent documents.
Specifically, to make 3 optical disks compatible, it is found necessary that, even when the objective lens is formed of the refractive lens or the diffractive lens, the optical magnifications of the objective lens for DVD and CD are made different. However, when, to assure the light amount, the optical magnification of the entire pick-up is made a predetermined value, or when the light source unit called 2LD1P which is in the market, is used, in the case where it is desired that the distance from the collimator to the light source is set at designer's discretion for DVD and CD, form the relationship of the pick-up space, it is necessary that the light is incident on the objective lens at an angle of light coincident to its optical magnification, in spite of the position of the light source. Accordingly, when the collimator or coupling lens is commonly used for DVD and CD, it is desired that the optical magnification of one collimator or coupling lens is different corresponding to the wavelength.